Partial-wave channel modes for a 2-terminal system

In traditional scattering theory, partial waves form a basis in which to decompose the asymptotic ( ) form of the full wavefunction into incoming and outgoing states of definite angular momentum . The basis functions are the cylindrical (in 2D) solutions to the free-space wave equation; the -matrix which takes incoming to outgoing waves can then be written in this basis [128]. Because there is only a single set of incoming channels and a single set of outgoing channels, this is equivalent to a scattering system (a stub) connected to a single `lead', with an infinite number of open channel modes. This contrasts the open two-terminal geometry we study, where we need to account for two new facts: 1) in the limit the potential no longer becomes angular-momentum-preserving, and 2) there are now distinct ways the particle can enter and exit the system, via different leads.

We define a `half-plane partial-wave basis' as the subset of the cylindrical
free-space
solutions which go to zero on the entire -axis.
This gives independent basis functions existing on either the left or right
side
of the -axis. The basis is expressed in terms of
Hankel functions[7] on either side:

where on the left () side is the angle from the negative -axis and on the right () side is the angle from the positive -axis (see Fig. 7.1a). The channel index is , and () refers to outgoing (incoming) travelling waves. We note that the s-wave is excluded because of the -axis barrier, leaving the first channel as the p-wave . Assuming the width of the barrier is finite and constant as (see Fig. 7.1a), then any wavefunction in the limit can be written as a sum of the above basis functions (this is discussed further in Appendix L). The separability of this basis in is directly analogous to the separability of conventional (constant-width) lead basis states [129,130,41] into a product of transverse modes and longitudinal travelling waves.

The corresponding outgoing (incoming) amplitude coefficients are () on the left and () on the right. Applying (7.5) to the aymptotic forms of the Hankel functions shows that a unit amplitude carries a flux of , independent of channel index . This results from the fact that all channels have the same asymptotic particle velocity, equal to that in free space. This contrasts the usual constant-width lead case where the non-zero transverse energy results in an -dependent asymptotic velocity, and thus care is needed to include a velocity factor in the normalisation [74,187,55].

Now we give a brief reminder of the recipe for the conductance
of a general completely-open single channel [20,65,55].
The channel is a 1D Fermi gas corresponding to the
longitudinal degree of freedom.
This can correspond to a conventional (constant-width) transverse
mode, to a Hankel (cylindrical) mode defined above, or indeed
to any situation where the Hamiltonian is separable into
(orthogonal) transverse modes and longitudinal motion in the asymptotic regime
^{7.6}.
For a 1D Fermi gas the rightward-moving density of states
per unit length (including spin) is
,
where is the (group) velocity in the channel.
The current carried
is
in the relevant energy range
,
giving a single `quantum' of conductance
.
The important point is that the velocity factors cancel,
giving the
*equipartition rule* that
an equal current is carried by all open channels regardless of
their channel velocities.

If the basis is chosen such that unit amplitude
coefficients carry equal fluxes
in all incoming and outgoing
channels, then flux conservation implies
the unitarity of the -matrix when written in this basis.
Then the results of the previous paragraph
generalise to the familiar Landauer formula